/**
 * Filters the message that is returned when a Webmention already exists and was updated.
 *
 * @param string $message 
 * @return string The filtered value.
 */
function my_webmention_update_message_callback( string $message ) {
    // Your code here.
    return $message;
}
add_filter( 'webmention_update_message', 'my_webmention_update_message_callback' );

apply_filters( 'webmention_update_message', esc_html__( 'You already sent a Webmention for this target. The existing Webmention has been updated.', 'webmention' ) )